This invention relates, in general, to the fabrication of a semiconductor structure, and more particularly, to the fabrication of a direct wafer bonded structure.
The direct wafer bonding of one semiconductor substrate to an insulating substrate or to an insulating layer formed on a semiconductor substrate has been reported in the past. Such structures are often called semiconductor-on-insulator (SOI) or dielectrically isolated direct wafer bonded structures. Typically, these direct wafer bonded structures use buried layers comprising diffused dopants or doped polysilicon. However, these buried layers have a disadvantage in that they have sheet resistance values on the order of 20 ohms/square. High sheet resistance values cause long RC time constants and hence undesirably long time delays. As a result, buried layers comprising high temperature metal nitrides such as titanium-nitride, vanadium-nitride, and tungsten-nitride are desirable because they have sheet resistance values on the order of 10 ohms/square and they are stable at temperatures in excess of 1100.degree. C.
However, when a patterned high temperature metal nitride layer is used for a buried layer, significant adhesion problems occur between the high temperature metal nitride layer and a dielectric layer formed over the high temperature metal nitride layer. This is particularly true when the layers are subsequently subjected to high temperature. Poor adhesion negatively impacts bonded wafer yield and device reliability. The direct wafer bonding process requires high temperature processing after the dielectric layer is formed over a high temperature metal nitride layer. Thus, there exists a need for a process for forming a dielectric layer over a high temperature metal nitride layer that provides good adhesion between the layers when the layers are subjected to high temperature.
Furthermore, direct wafer bonded structures often incorporate thick dielectric layers having low thermal conductivities between the bonded substrates. Thick dielectric layers that have low thermal conductivities result in poor thermal dissipation, which complicates the large scale integration of logic devices and power devices. Thus, there also exists a need for a direct wafer bonded structure that has good thermal conductivity.